Try this:
// categoryParentID is defined above
var catByParent = CategoryInfoProvider.GetCategories().WhereEquals("CategoryParentID", categoryParentID).Columns("CategoryID");
var documentCategories = DocumentCategoryInfoProvider.GetDocumentCategories().WhereIn("CategoryID", catByParent).Column("DocumentID");
var docs = DocumentHelper.GetDocuments().WhereIn("DocumentID", documentCategories);